Hydrostatic drive or transmission systems require careful control of system hydraulic pressures and provisions for changing direction of hydraulic fluid flow when the driven is shifted from forward to reverse drive and vice-versa. Overpressures in the system must be carefully controlled and high or excessive pressures must be rapidly reduced to prevent damage to the component parts. In order to provide for overpressure protection, high pressure relief valves are generally provided for in such systems. Shuttle valves are also provided to enable communication of hydraulic fluid from either side of the hydraulic circuits upon a reverse in hydraulic fluid pressurization.